Didn't the GP already answer the question you asked of them?
>> It was a purely defensive move to prevent anyone else from acquiring or partnering with Goodreads, because their database of books and reviews could be used to instantly start competing with Amazon's book business. Amazon snuffed out that threat of competition in an instant.
A catalog of user reviews is a pretty important feature for a successful eCommerce site. To the point where I've seen some "review sharing" between non-Amazon sites (the specific case was an eCommerge platform showing reviews from their own site and the manufacturer's direct-to-customer site). Goodreads was ready-made to provide that to a competitor, which would give them a shortcut around that moat.